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PLANT TISSUES:

PLANT TISSUES By: Mahalia Cornelio

INTRODUCTION :

INTRODUCTION Plants are composed of three major organ groups: roots, stems and leaves. As we know from other areas of biology, these organs are comprised of tissues working together for a common goal (function). In turn, tissues are made of a number of cells which are made of elements and atoms on the most fundamental level. In this section, we will look at the various types of plant tissue and their place and purpose within a plant.

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Plant tissues are characterized and classified according to their structure and function. The organs that they form will be organized into patterns within a plant which will aid in further classifying the plant. A good example of this is the three basic tissue patterns found in roots and stems which serve to delineate between woody dicot , herbaceous dicot and monocot plants.

Meristematic Tissues:

Meristematic Tissues Tissues where cells are constantly dividing are called meristems or meristematic tissues. These regions produce new cells. These new cells are generally small, six-sided boxlike structures with a number of tiny vacuoles and a large nucleus, by comparison. Sometimes there are no vacuoles at all.

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As the cells mature the vacuoles will grow to many different shapes and sizes, depending on the needs of the cell. It is possible that the vacuole may fill 95% or more of the cell’s total volume.

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There are three types of meristems : Apical Meristems Lateral Meristems Intercalary Meristems

Apical meristems:

Apical meristems Apical meristems are located at or near the tips of roots and shoots. As new cells form in the meristems , the roots and shoots will increase in length. This vertical growth is also known as primary growth. A good example would be the growth of a tree in height.

Each apical meristem will produce embryo leaves and buds as well as three types of primary meristems: protoderm, ground meristems, and procambium. These primary meristems will produce the cells that will form the primary tissues.:

Each apical meristem will produce embryo leaves and buds as well as three types of primary meristems : protoderm , ground meristems , and procambium . These primary meristems will produce the cells that will form the primary tissues.

Apical meristems:

Apical meristems

Lateral meristems:

Lateral meristems Lateral meristems account for secondary growth in plants. Secondary growth is generally horizontal growth. A good example would be the growth of a tree trunk in girth. There are two types of lateral meristems to be aware of in the study of plants. The vascular cambium, the first type of lateral meristem , is sometimes just called the cambium.

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The cambium is a thin, branching cylinder that, except for the tips where the apical meristems are located, runs the length of the roots and stems of most perennial plants and many herbaceous annuals. The cambium is responsible for the production of cells and tissues that increase the thickness, or girth, of the plant.

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The cork cambium, the second type of lateral meristem , is much like the vascular cambium in that it is also a thin cylinder that runs the length of roots and stems. The difference is that it is only found in woody plants, as it will produce the outer bark. Both the vascular cambium and the cork cambium, if present, will begin to produce cells and tissues only after the primary tissues produced by the apical meristems have begun to mature.

Lateral meristems:

Lateral meristems

Intercalary meristems :

Intercalary meristems Intercalary meristems are found in grasses and related plants that do not have a vascular cambium or a cork cambium, as they do not increase in girth. These plants do have apical meristems and in areas of leaf attachment, called nodes, they have the third type of meristematic tissue. This meristem will also actively produce new cells and is responsibly for increases in length. The intercalary meristem is responsible for the regrowth of cut grass .

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Sometimes the tissues are composed of the same type of cells throughout, or sometimes they are mixed. There are simple tissues and complex tissues to consider, but we will start with the simple tissues for the sake of discussion.

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There are other tissues in plants that do not actively produce new cells. These tissues are called nonmeristematic tissues. Nonmeristematic tissues are made of cells that are produced by the meristems and are formed to various shapes and sizes depending on their intended function in the plant.

Parenchyma tissue :

Parenchyma tissue Parenchyma tissue is composed of cells (parenchyma cells) that are thin-walled, more or less isodiametric , and alive at maturity. Parenchyma cells function in the manufacture of food for the plant (most of the chloroplast-containing cells of the leaf are parenchyma cells) and in the storage of materials within the plant body.

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Thin-walled parenchyma cells from the tuber of an Irish (white) potato, stained with toluidine blue. The granules within the individual cells are starch grains.

Close-up of a single parenchyma cell from potato, showing the starch grains stored within the cell.:

Close-up of a single parenchyma cell from potato, showing the starch grains stored within the cell.

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The cells of parenchyma are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other and are usually stuffed with plastids. In areas not exposed to light, colorless plastids predominate and food storage is the main function. The cells of the white potato are parenchyma cells

Sclerenchyma tissue:

Sclerenchyma tissue Sclerenchyma tissue is composed of cells ( sclerenchyma cells) that have extremely hard, thick walls. In fact, the cell walls of sclerenchyma cells are so thick that, at maturity, the cell is completely cut off from the extracellular environment and dies. Two general types of sclerenchyma cells are recognized: sclereids , which may be more or less isodiametric or may be branched, and fibers, which are greatly elongate cells.

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The gritty texture of pears is caused by groups of sclereids (often called "stone cells") that are embedded in the parenchyma tissue of the fruit's flesh.

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Transfer cells- specialized for short distance transfer of solutes between cells; have secondary cell walls; they are inner extensions of wall that increase surface area. Transfer cells occur in areas of high solute transport, such as secretory tissues, which release substances that are produced within the protoplasm and are moved outside, i.e., nectar cells, mucilage in sundews, and resins.

Collenchyma tissues:

Collenchyma tissues Collenchyma tissues are mainly found under the epidermis in young stems in the large veins of leaves. The cells are composed of living, elongated cells running parallel to the length of organs that it is found in. Collenchyma cells have thick cellulose cell walls which thickened at the corners.

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Intercellular air spaces are absent or very small. The cells contain living protoplasm and they sometimes contain chloroplasts. Functions: the collenchyma serve as supporting and strengthening tissue , in collenchyma with chloroplasts, photosynthesis takes place.

Collenchyma tissues:

Collenchyma tissues

Epidermal tissue :

Epidermal tissue Epidermal tissue is a single layer of cells, (except in velamen , pepperomia and rubber plants) that covers the plant body. Functions: acts as a buffer between the environment and the internal plant tissues; absorption of water and minerals primarily in the root region.

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on the stem and leaves generally covered with a cutin which prevents evaporation, secretion of cuticle (waterproof; made of cutin = fatty substances) can form a barrier resistant to bacteria and fungi. can prevent leaching of materials in or out of the surface. recognition: lectin on epidermal surface can permit recognition of other types of cells- sometimes of other organism such as invading fungi.

Root hairs:

Root hairs Root hairs- absorb water and the material it carries ( minerals). They increase absorptive surface area of roots Although single celled, they can be viewed by the eye on rapidly growing radishes. An important surface over which plants absorb most of their water and nutrients. They are also directly involved in the formation of root nodules in legume plants. .

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Root hair cells are outgrowths at a tip of the plants roots. Root hair cells vary between 5 and 17 micrometres in diameter, and 80 to 1,500 micrometres in length. They are found only in the zone of maturation, and not the zone of elongation, possibly because any root hairs that arise are sheared off as the root elongates and moves through the soil

Root hairs:

Root hairs

Leaf hairs:

Leaf hairs Leaf hairs ( trichomes )- 1-2 cells; important in boundary layer and in defense. They reflect light to protect against overheating and excessive water loss. This is an incredibly important function for plants in dry regions where excess light may lead to photobleaching of pigments and excess absorption of light would overheat the tissue.

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The layer acts to hold in a layer of humidity 'trapped' between the epidermis and the tips of the trichomes . As water diffuses from a region of high to low density, less water will escape from the stomata to the outside air since the air layer outside the stomata is 'moist' due to its entrapment by the trichomes .

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This layer also prevent air moving directly against the stomata which would encourage water loss. Its' rough surface breaks up wind currents .

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When in high density they act to deter herbivory by small animals (image being an insect walking through a forest of these spiky trichomes ). Dependent on species they may have barbs or tips that give off nasty compounds when hit. Some have rigid barbs that impale insects dropping onto them, as with flying insects)

Leaf hairs:

Leaf hairs

Stomata:

Stomata Created by guard cells; most abundant on underside of leaves. Regulate diffusion of CO2 into the leaf for photosynthesis as well as regulate loss of water from the leaf. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening.

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Stomata respond to turgor pressure *stomata open when K + and water enter, thereby increasing turgor pressure in guard cells, thus pushing them apart *stomata close when K + and water leave the guard cells, thus leaving them flaccid and closing upon each other.

Stomata:

Stomata

Salt glands:

Salt glands Salt glands- dump sites for the excess salt absorbed in water from the soil; help plants adapt to life in saline environments; a crust of salt forms on leaves which tastes bad and the white surfaces act to reflect light.

Salt glands:

Salt glands

Vascular tissue:

Vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem.

Xylem:

Xylem The xylem transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well.

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Functions: transport water and dissolved substances support the physical structure can act in food storage

Xylem:

Xylem

Phloem:

Phloem Phloem is the living tissue that carries organic nutrients (known as photosynthate ), in particular, sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark transports dissolved organic materials -sieve elements are the conducting cells of phloem.

There are two kind of sieve elements::

There are two kind of sieve elements: Sieve cells Sieve tube members

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Sieve cells- occur in nonflowering plants; long with tapered overlapping ends; associated with albuminous cells, which help regulate the sieve cells' activities. Sieve tube members - arranged end to end in sieve tubes; larger pores than sieve cells; concentrated along contacting end walls of adjacent sieve tube members .

Phloem:

Phloem

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Sieve cells Sieve tube members

Vessels:

Vessels predominate in angiosperms; have short, wide, thick secondary cell walls are dead, hollow cells; lack end walls; have a large diameter, therefore water movement through them is rapid; the wall thickenings in vessels are pitted as you can see from the image on the right.

Tracheids:

Tracheids predominate in conifers; long, slender cells with tapered, overlapping ends; water moves upward from tracheid to tracheid through pit pairs, thus preventing large gas bubbles from forming and thus no cavitation during freezing/defrosting periods.